1. Field of the Invention
The present invention relates to a disc playing apparatus for playing a disc on which digital data to be transmitted at a predetermined transmission rate is recorded as a subcode, in addition to a coded information signal such as a digital audio signal.
2. Description of Background Information
A subcode carrying play control information for example is recorded on recording discs such as the digital audio disc called CD (compact disc), the disc called CD-V having an area in which only digital audio signal is recorded and an area in which a video signal is recorded in addition to the digital audio signal, and the video disc called LDD on which digital audio signal of CD format is recorded by a frequency multiplex process. The subcode is constituted by channels P, Q, R, S, T, U, V, and W in which channels P and Q are used as the control signal of the disc player. Specifically, the P channel bits of the subcode are used for distinguishing intervals between two music selections from areas in a music selection. A 0 is set to the P channel for the area in a music selection and a 1 is set for the interval between two music selections. The Q channel bits are used to indicate the music selection number of the music selection being played, and the time information such as the minutes, seconds, and the frame number which are measured from the beginning of the first one of the recorded music selections. The bits indicating the minutes, seconds and the frame number from the beginning of the first music selection are utilized as a block identification code for identifying each of the blocks designated as subcoding frame.
The channels R through W are vacant channels called user's bit, and various manners of utilization of these channels such as the recording of picture information are being considered. A standard of the recording format for recording picture information has been already proposed.
On the other hand, a standard called MIDI (Musical Instrument Digital Interface) has been developed so as to enable a music performance using a plurality of sound sources by combining a plurality of electronic musical instruments such as the music synthesizer or the electronic piano. A term "MIDI apparatus" is used for designating an electronic musical instrument which includes a hardware according to MIDI standard and has a function to transmit and receive a data format signal (referred to as MIDI signal hereinafter) as a musical instrument control signal of a defined form carrying musical information.
The MIDI signal is a serial data having a transfer rate of 31.25 Kbaud, and one byte data is constituted by 10 bits including 8-bit data and start and stop bits respectively having one bit.
In order to designate the sort of the transmitted data and the MIDI channel, at least one status byte and one or two data bytes leaded by the status byte are combined, forming a message as the musical information. Thus, one message is constituted by 1 through 3 bytes, and the transfer period of 320 through 960 .mu. sec is required.
As an example of such a message, the structure of the note-on message is shown in FIG. 1.
The note-on message in the status byte is a command corresponding to an operation of pressing a key of the keyboard, and used in a pair together with a note-off message which corresponds to an operation of releasing the key of the keyboard. The "note-number" in the data byte # 1 is used to designate one of 128 stages which are allotted to the keys of the keyboard with its center positioned on the "central do" of the musical scale provided by the sound source. The "velocity" in data byte # 2 is a number corresponding to the velocity of operation of the keyboard, and indicates the strength of the sound to be generated. Upon receipt of the note-on message, a MIDI apparatus generates a note of the designated musical scale at the designated strength, and stops the generation of the note when it receives the note-off message.
Not only picture information mentioned before, the MIDI signal can also be recorded in the user's bit of the subcode. By recording picture information and the MIDI signal in the user's bit, it is possible to control one or more additionally provided MIDI apparatuses in a predetermined order in addition to the reproduction of pictures and sounds obtained by the audio and video signals reproduced by the disc player. Therefore, various possibility of the utilization of this arrangement, such as the construction of an audio visual system including electronic musical instruments and capable of a program reproduction with full of presence, or the application to the production of educational softwares, are being considered.
An example of the construction of data format with the channels R through W of the subcode, i.e., the user's bits is shown in FIG. 2. As shown in this figure, one symbol is formed of six bits belonging to the channels R through W respectively. Ninety-eight symbols are treated as one block. Among these 98 symbols, 2 symbols are used as the synchronizing signal for subcoding frames, and 24 symbols obtained by dividing the remaining 96 symbols by 4 are treated as a minimum unit of the data, i.e. a Pack, and an image processing instruction or a MIDI signal group is constituted by this minimum data unit.
The symbol 0, i.e., the first one of the 24 symbols, represents the Mode. For instance, if the symbol 0 is "001001", it represents the TV graphics mode. On the other hand, if it is "111111" it indicates the MIDI signal mode. The symbol 1 following the symbol 0 is the "Instruction" indicating the sort of the instruction in the case of the picture processing instruction. The symbols 3 and 4 form the parity Q operating as an error correction code. The symbols from the symbol 4 through the symbol 19 form a data field, which carries the color information or the MIDI signal. The symbols 20 through 23 together form a parity P, that is an error correction code for protecting information in the pack.
As described above, picture information or the like is recorded on a disc in addition to the digital audio signal by using the user's bits of the subcode. Conventional disc players for playing such a disc have been configured to successively play the information recorded by the user's bit of the subcode in accordance with the progress of the play of the digital audio signal.
However, there are cases where it is desired to perform the so-called slow playback in which the playing speed of the information recorded by using the user's bit of the subcode is slower than the playing speed during the play of the digital audio signal or the so-called a fast (or multiplied-speed) playback in which the playing speed is higher than the playing speed during the play of the digital audio signal. With the conventional player, is has been impossible to perform a fast playback of the information recorded by using the user's bit of the subcode. Furthermore, it has been necessary to appropriately command a temporary interruption of the playing operation i.e. the so-called pause mode, by a manual operation, in order to perform the slow playback. Thus, complicated operations have been required in the case of conventional players.